What substance can lead to "blue-baby" syndrome when present at high levels in water?

The correct answer is nitrates, as high levels of nitrates in drinking water can lead to "blue-baby" syndrome, also known as methemoglobinemia. This condition occurs primarily in infants, where the presence of elevated nitrate levels in their system interferes with the ability of red blood cells to carry oxygen effectively. When nitrates are ingested, they can be converted to nitrites in the body. Nitrites bond with hemoglobin, forming methemoglobin, which cannot transport oxygen. This results in a characteristic bluish color of the skin due to a lack of oxygen in the bloodstream.

Nitrates can often enter drinking water supplies through agricultural runoff, particularly from fertilizers and animal waste. It is important to monitor and manage these levels to ensure the safety of water sources for infants and young children, making awareness of the risks associated with high nitrate levels vital for water treatment professionals.